Silicone resins

ABSTRACT

A silicone resin, curable to a resin of low coefficient of thermal expansion, high glass transition temperature and high modulus, has the empirical formula (R 3 SiO 2 ) a (R 2 SiO 2/2 ) b (RsiO 3/2 ) c (SiO 4/2 ) d  wherein each R is a hydrocarbon or substituted hydrocarbon group or a hydrogen atom; and a=0.02 to 0.8; b=0 to 0.4; and c+d=0.2 to 0.98, where a+b+c+d=1.0, characterized in that at least 2 mole % of the siloxane units in the resin are of the formula R′ 3 SiO 1/2  RR′2Sio1/ 2  or R′ 2 SiO 2/2  wherein each R′ is an alkenyl group.

This invention relates to silicone resins curable by additionpolymerisation and/or hydrosilylation, to methods of preparation of thecurable resins, and also to processes for curing the resins and to curedresins produced thereby.

BACKGROUND TO THE INVENTION

There is an increasing need for resins with good dimensional stability(low coefficient of thermal expansion (CTE), high glass transitiontemperature Tg and high modulus) and moisture and heat resistance over awide temperature range. There is a particular need for resins which canbe applied in a curable state and which can be cured in a thick sectionand are thus suitable for encapsulating delicate substrates, for exampleas underfill for microelectronic device packaging, as matrix resin incomposites, and also in coatings such as wafer level and solar panelcoatings, in planarization layers for Flat Panel Displays and inphotonic devices.

Silicone resins have excellent heat resistance and are moisturerepellent but typically have a CTE in the range 110 to 350 ppm/° C.,compared to 50 to 120 ppm/° C. for most organic polymers and resins. Thepresent invention seeks to produce silicone resins having reduce CTE inthe cured state so that they are more suitable for the uses listed above

U.S. Pat. No. 6,124,407 describes a silicone composition comprising (A)100 parts by weight of a polydiorganosiloxane containing an average ofat least two silicon-bonded alkenyl groups per molecule; (B) 75 to 150parts by weight of an organopolysiloxane resin containing an average offrom 2.5 to 7.5 mole percent of alkenyl groups; (C) anorganohydrogenpolysiloxane having an average of at least threesilicon-bonded hydrogen atoms per molecule in an amount to provide fromone to three silicon-bonded hydrogen atoms per alkenyl group incomponents (A) and (B) combined; (D) an adhesion promoter in an amountto effect adhesion of the composition to a substrate; and (E) ahydrosilylation catalyst in an amount to cure the composition. Thecomposition is useful as an encapsulant in chip scale packages.

Siloxane resins containing at least two alkenyl groups, present forexample as dimethylvinylsilyl units, are described in U.S. Pat. No.5,777,047 and U.S. Pat. No. 5,468,828. These siloxane resins are used inminor amounts to cure a polydiorganosilane to an elastomer, rather thanto form a rigid thermoset resin.

DESCRIPTION OF THE INVENTION

According to one aspect of the present invention, a curable siliconeresin has the empirical formula(R₃SiO_(1/2))_(a)(R₂SiO_(2/2))_(b)(RSiO_(3/2))_(c)(SiO_(4/2))_(d)wherein each R is a hydrocarbon or substituted hydrocarbon group or ahydrogen atom; and a=0.02 to 0.8; b=0 to 0.4; and c+d=0.2 to 0.98, wherea+b+c+d =1.0, characterized in that at least 2 mole % of the siloxaneunits in the resin are of the formula R′₃SiO_(1/2), RR′2SiO1_(/2) or orR′₂SiO_(2/2), wherein each R′ is an alkenyl group.

The invention also includes a process for the preparation of a curedheat resistant silicone resin having a low coefficient of thermalexpansion, characterised in that a curable silicone resin as describedabove is reacted with a curing agent having at least one functionalgroup reactive with the alkenyl group R′. The cured resin containingbranching derived from R′3SiO1_(/2) or RR′₂SiO_(1/2) or R′₂SiO_(2/2)siloxane units has low CTE, high Tg and modulus and high thermalstability, and increased mechanical strength compared to a resin havinga high proportion of SiO_(4/2) units.

The groups R′ in the siloxane units R′₃SiO_(1/2) or RR′₂SiO_(1/2) orR′₂SiO_(2/2) are generally alkenyl groups having 2 to 8 carbon atoms,and are preferably vinyl groups although allyl or hexenyl groups arealternatives. Preferably at least 10 mole %, more preferably at least 15mole %, of the siloxane units of the curable silicone resin areR′₃SiO_(1/2) or RR′₂SiO_(1/2) or R′₂SiO_(2/2), groups, most preferablyR′₃SiO_(1/2) groups such as Vi₃SiO_(1/2) groups, where Vi representsvinyl.

The groups R in the siloxane units RR′₂SiO_(1/2) are most preferablymethyl groups but can be other alkyl groups having up to 4 carbon atoms,for example ethyl groups, or aryl groups, particularly phenyl. Thesiloxane units RR′₂SiO_(1/2) can for example be divinylmethylsiloxy ordivinylphenylsiloxy units.

The groups R in the siloxane units of the formula RSiO_(3/2) (T units)are selected from hydrocarbon groups, substituted hydrocarbon groups andhydrogen atoms (Si—H groups). Examples of hydrocarbon groups are alkylgroups, preferably having up to 4 carbon atoms, most preferably methylgroups, or aryl groups, particularly phenyl. Examples of substitutedhydrocarbon groups are haloalkyl, alkoxyalkyl and chlorophenyl groups.The groups R in the T units can be the same or different. Preferably atleast 80 mole % of the siloxane units of the resin are selected fromR′₃SiO_(1/2), RSiO3_(/2) and SiO_(4/2) units. Similar groups R can be inthe R₂SiO_(2/2) units (D units) if any are present, but the resinpreferably contains no more than 40 mole %, more preferably less than 20mole %, D units. The resin may optionally contain R′R₂SiO_(1/2) orR₃SiO_(1/2) units where each R is other than alkenyl, although forstability and safety reasons it is preferred that no more than twohydrogen atoms, most preferably no more than one hydrogen atom, arebonded to each Si atom.

In one preferred embodiment of the invention, at least 5 mol%, mostpreferably at least 20%, of the siloxane units of the resin are of theformula ArSiO_(3/2) where Ar represents an aryl group up to 50 or even70 mol % ArSiO_(3/2) units. The aryl groups Ar in the siloxane T unitsof the formula ArSiO_(3/2) are preferably phenyl groups, althoughnaphthyl or tolyl groups are alternatives. The aryl groups may enhancethe thermal stability of the cured silicone resin.

In one process according to the invention for the preparation of acurable silicone resin, at least one chlorosilane of the formulaR′₃SiCl, RR′2SiCl or R′₂SiCl₂ is reacted with at least one chlorosilaneof the formula RSiCl₃ and/or SiCl₄ and/or an alkoxysilane of the formulaRSi(OY)₃ or Si(OY)4 and optionally a chlorosilane of the formulaR₂SiCl₂. For example at least one chlorosilane of the formula R′₃SiCl orRR′₂SiCl can be reacted with at least one chlorosilane of the formulaRSiCl₃ or SiCl₄ in the presence of water and a dipolar aprotic solventwhich is at least partially miscible with water, for exampletetrahydrofuran (THF), dioxane or a ketone containing 4 to 7 carbonatoms such as methyl isobutyl ketone (MIBK), methyl ethyl ketone ormethyl isoamyl ketone.

A curable silicone resin according to the invention can alternatively beprepared by reaction of a mixture comprising an alkoxysilane of theformula R′₃SiOY, RR′2SiOY or R′₂Si(OY)₂ with an alkoxysilane of theformula RSi(OY)₃ and/or _(Si)(OY)₄ where Y is an alkyl group having 1 to6 carbon atoms, preferably methyl or ethyl, in the presence of water, ahydrolysis catalyst such as an inorganic acid or base, and preferably anorganic solvent for the reaction product such as a ketone or aromatichydrocarbon. In a further alternative process, a curable silicone resinaccording to the invention containing siloxane units of the formulaSiO_(4/2) can be prepared by dripping an alkyl silicate(tetraalkoxysilane) into a mixture of aqueous HCl of concentration atleast 5% by weight and a disiloxane of the formula (R′₃Si)2O or(RR′₂Si)2O, or a chlorosilane of the formula R′₃SiCl or RR′₂SiCl, or analkoxysilane of the formula R′₃SiOY or RR′2SiOY.

In another preferred process for preparing a silicone resin according tothe invention containing siloxane units of the formula SiO_(4/2) asilicone resin of the formula(R₃SiO_(1/2))_(a)(R₂SiO_(2/2))_(b)(RSiO_(3/2))_(c)(SiO_(4/2))_(d)wherein each R is a hydrocarbon or substituted hydrocarbon group or ahydrogen atom; and a=0.02 to 0.8; b=0 to 0.4; and c+d=0.2 to 0.98, wherea+b+c+d=1.0, at least 2 mole % of the siloxane units in the resin beingof the formula R′₃SiO_(1/2), RR′₂SiO_(1/2) or R′₂SiO_(2/2), wherein eachR′ is an alkenyl group, in which some of the_(1/2), siloxane units areof the formula HSiO_(3/2), is treated in solution with a base tocondense at least some of the HSiO_(3/2) units to form SiO_(4/2) units.The base is preferably a solution of an alkali metal salt of a weak acidsuch as a carboxylic acid, for example sodium acetate, sodium hydrogenphosphate or sodium tetraborate. An aqueous and/or organic solventsolution can be used. A preferred solvent mixture comprises water and adipolar aprotic solvent which is at least partially miscible with water,for example a ketone having 4 to 7 carbon atoms, as described above, ora cyclic ether such as tetrahydrofuran or dioxane. Alternatively thebase may comprise an amine, preferably a tertiary amine, particularly atrialkyl amine such as triethylamine or tripropylamine, or alternativelypyridine or dimethylaminopropanol. The base can for example be anaqueous solution of triethylamine. A tertiary amine can act as both baseand as a dipolar aprotic solvent, so that one base reagent comprises asolution of an alkali metal salt of a weak acid in a solvent mixture ofwater and a tertiary amine. The base treatment causes hydrolysis of someof the Si—H groups of the resin to Si—OH groups and subsequentcondensation of the Si—OH groups to Si—O—Si linkages, thus converting atleast some of the HSiO_(3/2) units to form SiO_(4/2) units.

In an alternative process for preparing a silicone resin according tothe invention containing siloxane units of the formula SiO_(4/2), asilicone resin comprising siloxane D T units of the formula R″SiO_(3/2)and HSiO_(3/2) is treated in solution with a base to condense at leastsome of the HSiO_(3/2) units to form SiO_(4/2) units, and the resultingresin solution is reacted with a chlorosilane of the formula R′₃SiCl orRR′₂SiCl.

The degree of conversion of HSiO_(3/2) units to SiO_(4/2) units can becontrolled by controlling the strength and concentration of the baseused to treat the resin, the time of contact between the resin and thebase and the temperature of the reaction. The base strength andconcentration and time and temperature of treatment are preferablysufficient to condense at least 30%, preferably at least 50%, up to 80%or 100%, of the HSiO_(3/2) units to SiO_(4/2) units. The temperature ofthe reaction with base can for example be in the range 0-140° C. Forexample, a 0.5M sodium acetate solution in aqueous MIBK will cause 50%conversion of HSiO_(3/2) units to SiO_(4/2) units at 100-110° C. inabout 1 hour. A 0.5M solution of sodium acetate in aqueous triethylaminewill cause 50% conversion at 25° C. in about 30-40 minutes.

The subsequent reaction of the resulting resin solution with achlorosilane of the formula R′₃SiCl or RR′₂SiCl converts most of theremaining Si—OH groups to Si—O—SiR′₃ groups or Si—O—SiRR′₂ groups. Theresin solution and chlorosilane are preferably reacted in the presenceof a disilazane, which aids in the reaction of Si—OH groups. Thedisilazane is preferably a disilazane of the formula RR′2Si—NH—SiRR′2,in which the groups R and R′ are the same as in the chlorosilaneRR′2SiCl, although an alternative disilazane can be used such astetramethyldisilazane or hexamethyldisilazane. The reaction ispreferably carried out under substantially anhydrous conditions in anorganic solvent, for example a ketone having 4 to 7 carbon atoms and/oran aromatic hydrocarbon such as toluene or xylene. The reaction can becarried out at a temperature in the range 0-140° C., preferably 20-80°C. The reaction serves to introduce R′₃Si— or RR′₂Si— groups into theresin and to reduce the level of Si—OH. The starting resin comprisingsiloxane T units may also comprises siloxane M units of the formulaR′₃SiO_(1/2) or RR′₂SiO_(1/2) since the reaction of the resin solutionwith chlorosilane and optionally disilazane may not always introducesufficient R′ groups to give the desired level of cure.

The curable resin of the invention can be a self-curable resin whichalso contains HSiO_(3/2) units and/or HR₂SiO_(1/2), H₂RSiO_(1/2) orHRSiO_(2/2) units. For example, in a preferred self-curable resin 10 to50 mol % of the siloxane units of the resin are HSiO_(3/2) units and 5to 40 mol % of the siloxane units of the resin are of the formulaR′₃SiO_(1/2) or RR′₂SiO_(1/2). Such a self-curable resin can be preparedby any of the processes described above using trichlorosilane HSiCl3 togenerate HSiO_(3/2) units. The proportion of HSiCl3 reacted is selectedto be sufficient to provide any HSiO_(3/2) units which are required forconversion to SiO_(4/2) units as well as the desired level of HSiO_(3/2)units to form a self-curable resin. A self-curable resin containingHR2SiO_(1/2) units can be prepared by reacting a base hydrolysed resinwhich comprises siloxane M units of the formula R′₃SiO_(1/2) orRR′₂SiO_(1/2) with a chlorosilane of the formula HR₂SiCl, preferably inthe presence of a disilazane such as tetramethyldisilazane. Aself-curable silicone resin containing HSiO_(3/2) units and/orHR2SiO1_(/2) units as well as R′₃SiO_(1/2) or RR′₂SiO_(1/2) units can becured to a heat resistant silicone resin having a low coefficient ofthermal expansion by heating in the presence of a catalyst containing aplatinum group metal.

The curable resin generally has a molecular weight of at least 500 up to300000 or even higher, for example in the range 1000 to 20000. Treatmentof the resin with a base to condense HSiO_(3/2) units to form SiO_(4/2)units generally increases the molecular weight of the resin.

The invention includes a curable resin composition comprising a curablesilicone resin as defined above and a curing agent having at least onegroup reactive with the alkenyl group R′. The curing agent preferablycontains at least one Si—H group and the curable composition preferablycontains a curing catalyst, particularly a catalyst containing aplatinum group metal. The curing agent can for example be a polysiloxanecontaining at least two Si—H groups, such as a polydimethylsiloxanehaving terminal HZ₂Si— groups where each Z is an alkyl group, preferablymethyl, or phenyl group, or a further H atom, for exampleHMe₂Si—(O—SiMe₂)₄—O—SiMe₂H, (e represents methyl) or apolymethylhydrogensiloxane such as 1,3,5,7-tetamethylcyclotetrasiloxane,or a silicone resin containing HZ2Si— groups and T or Q units, forexample a low molecular weight MQ resin containing HMe₂Si— groups suchas (HMe₂SiO_(1/2))₈(SiO_(4/2))₈ or a MT resin such as(HMe₂SiO_(1/2))₃SiO_(3/2)Ph, where PH is phenyl. The curing agent canalternatively be an organic compound containing SiH groups, particularlyan aryl compound of the formula HX₂Si—Ar—SiX₂H, in which Ar is asubstantially nonflexible linkage including at least one para-arylenemoiety, for example p-phenylene or 4,4′-biphenylene, and each X is ahydrocarbon or substituted hydrocarbon group or a hydrogen atom. Thegroups HX₂Si— can for example be HMe₂Si— groups, H₂MeSi— groups or H3Si—groups; for example the curing agent can be1,4-bis(dimethylsilyl)benzene, 1,4-bis(methylsilyl)benzene or1,4-bis(silyl)benzene.

The curing agent is preferably used in an approximately stoichiometricamount, for example 70 to 150% of stoichiometric, based on the alkenylgroups in the curable silicone resin. The concentration of Si—H groupsin the curing agent is preferably such that the curing agent is presentat less than 200% by weight, most preferably less than 100% by weight,based on the curable silicone resin. The curable resin composition isgenerally not based on long chain polydiorganosiloxanes, and preferablyless than 40%, most preferably less than 20%, of the silicon atoms inthe curable resin composition are present as Me₂SiO_(2/2) units or otherdialkylsiloxane units.

The curing catalyst is preferably aplatinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex which canbe used at 20 to 200, for example about 50, parts per million Pt basedon the SiH-containing resin (mol/mol). Alternative curing catalysts canbe used, for example chloroplatinic acid or an analogous rhodiumcompound.

The silicone resin composition may additionally contain an inhibitor forthe curing reaction, an adhesion promoter that improves unprimedadhesion of the compositions to substrates commonly employed in theconstruction of electronic devices, and/or one or more fillers orpigments. Examples of inhibitors are 3-methyl-3-penten-1-yne and3,5-dimethyl-3-hexen-1-yne; acetylenic alcohols such as3,5-dimethyl-1-hexyn-3-ol, 1-ethynyl-1-cyclohexanol and2-phenyl-3-butyn-2-ol; maleates and fumarates, such as the well knowndialkyl, dialkenyl and dialkoxyalkyl fumarates and maleates;cyclovinylsiloxanes; and benzyl alcohol. Examples of adhesion promotersare an organosilicon compound containing at least one lower alkenylgroup or at least one silicon-bonded hydrogen atom and at least oneepoxy group, silanes and bis-silylhydrocarbons containing a plurality ofsilicon-bonded alkoxy groups and at least one vinyl group, an alkenylisopropenoxysilane or a product of the partial hydrolysis-condensationthereof, or an organosilane containing at least one alkoxy group and atleast one epoxy, methacryloxy or acryloxy group. Examples of preferredfillers include fused silica (fused quartz), alumina, boron nitride andaluminium nitride.

The invention includes a process for the preparation of a cured heatresistant silicone resin having a low coefficient of thermal expansion,characterised in that a curable silicone resin as described above isreacted with a curing agent having at least one functional groupreactive with the alkenyl group R′, preferably a curing agent containingat least one Si—H group. The curing process is preferably carried out inthe presence of a catalyst containing a platinum group metal. In analternative process a self-curable silicone resin as described above isheated in the presence of a catalyst containing a platinum group metal.The curing reaction is generally carried out at a temperature of atleast 50° C., preferably at least 100° C., for example in the range 150to 300° C., particularly 150 to 200° C.

The molecular weight of the curable resin can be controlled so that itis a flowable or a solid resin at room temperature. In a processaccording to the invention for encapsulating a substrate, the substrateis encapsulated in a curable silicone resin composition according to theinvention and the resin is then cured. Such a process can be used forencapsulating delicate substrates, particularly for microelectronicdevice packaging in processes such as Flip Chip Underfill, No FlowFluxing Underfill or moulding encapsulation, and may for example replaceepoxy or polyimide resins in such applications.

In a process according to the invention for coating a substrate, acurable silicone resin composition according to the invention is appliedas a thin film to a substrate before being cured. Such a process can beused in coatings such as wafer level and solar panel coatings, inplanarization layers for Flat Panel Displays and in photonic devices.Good, quality thin films between 600 nm to 100 μm thick can be produced,as can thick free-standing films several mm. thick.

A curable silicone resin according to the invention can be used toimpregnate a fibrous substrate in the production of composites. At leastone layer of fibrous material is impregnated with a curable siliconeresin composition and the resin is cured as described above, optionallyafter assembly of several impregnated layers to form a laminate, forexample in the production of printed circuit boards.

Cured silicone resins according to the invention are generally heatresistant and have a coefficient of thermal expansion of 120 ppm/° C. orbelow, for example 30 to 120 ppm/° C. The CTE can be further reduced bythe incorporation of a low CTE filler in the resin, for example silica,alumina or mica. The level of filler can for example be up to 1000% byweight based on the silicone resin, preferably at least 5% up to 500%,for example 25 to 80% by weight of the total silicone resin curablecomposition. The filler is mixed with the curable resin according to theinvention before curing. Incorporation of a low CTE filler can reducethe CTE of the filled resin below 50 ppm/° C., even to 10 ppm/° C. orbelow.

In a preferred process according to the invention, the cured siliconeresin is subsequently further heat cured at a temperature in the range300 to 500° C. The further heat curing at 300-500° C. producescrack-free cured resin exhibiting enhanced thermo-mechanical propertiessuch as higher Young's modulus, even lower CTE, higher plateau modulus(the minimum value of the Young's modulus over a temperature range of−100 to +300° C. including the glass transition temperature Tg,most-often within a plateau region at temperatures higher than Tg) andgood retention of film quality and strength.

The further heating step at 300-500° C. is preferably carried out in anon-oxidising atmosphere, for example it can be carried out under aninert gas such as nitrogen. Most preferably the further heating step at300-500° C. is carried out in the presence of an amine which is in thevapour state at the temperature of the further heating step. The amineis, preferably a tertiary amine; it can for example be a tertiary amineof the formula NZ₃, where each Z represents an alkyl group having 1 to 4carbon atoms.

The Young's and plateau moduli of the resins of the invention cured at50 to 300° C. and subsequently further heat cured at a highertemperature in the range 300 to 500° C. are generally above 1 Gpa, forexample 1.4 to 3.4 GPa. The fall in modulus from the Young's modulus tothe plateau modulus is generally below 20% and may be as low as 3%. Bothfree-standing filns and coatings have high quality and strength.

The invention is illustrated by the following Examples:

EXAMPLE 1 Preparation of M^(Vi3) _(0.21) T^(Ph) _(0.79) resin

62.5 g (0.29 mol) of phenyltrichlorosilane and 14.3 g (0.10 mol) oftrivinylchlorosilane were mixed into 80 ml of MIBK, and added dropwiseinto a solution consisting of 80 ml of a 1M HCl aqueous solution, 120 mltoluene and 160 ml MIBK at room temperature over a 1 h period. Themixture was then refluxed for another 3 hours under constant stirring.The aqueous layer was poured off and the organic layer was washed fourtimes with water until neutral pH. Removal of residual water byanhydrous NaSO₄, and stripping off the solvent led to 53g of a liquidbeing highly soluble in common organic solvents. The M^(Vi3)_(0.21)T^(Ph) _(0.79) composition of this resin was determined by ²⁹Siand ¹³C NMR spectroscopy (OH wt %=5%).

EXAMPLE 2 Preparation of M^(Vi3) _(0.23)M^(HMe2) _(0.13)T^(Ph) _(0.64)resin

52 g of M^(Vi3) _(0.21)T^(Ph) _(0.79) prepared according to example 1,were re-dissolved into 70 ml of anhydrous toluene and 10.4 g (72.0 mmol)of trivinylchlorosilane and 9.6 g (72.0 mmol) of1,1,3,3-tetramethyldisilazane were added. The mixture was stirred at 50°C. for 2 hours. After addition of 150 ml of distilled water, the organiclayer was collected and washed four times with water until neutral pH.The mixture was treated by anhydrous NaSO₄ to remove residual water byfurther filtration and volatiles were stripped off leading to 37 g of astraw yellow liquid (2,560 cP neat, or 20 cP with1,4-bis(dimethylsilyl)benzene cross-linker at 27.4° C.). The M^(Vi3)_(0.23)M^(HMe2) _(0.13)T^(Ph) _(0.64) composition of this resin wasdetermined by ²⁹Si and ¹³C NMR spectroscopy (OH wt %<1.5%; Mn=930;Mw=1,309).

EXAMPLE 3 Preparation of M^(Vi3) _(0.25)T^(H) _(0.75) resin

70.22 g (53.3 mmol) of trichlorosilane and 25.0 g (172.8 mmol) oftrivinylchlorosilane were mixed into 120 ml of MEBK, and added dropwiseinto a solution consisting of 120 ml of a 1M HCl aqueous solution, 180ml toluene and 240 ml MIBK at room temperature over a 1 h period. Themixture was then refluxed for another 1.5 hours at 110° C. underconstant stirring. The aqueous layer was poured off and the organiclayer was washed four times with water until neutral pH. Removal ofresidual water by anhydrous NaSO₄, and stripping off the solvent led to43 g of a light yellow liquid being highly soluble in common organicsolvents. The M^(Vi3) _(0.25)T^(H) _(0.75) composition of this resin wasdetermined by ²⁹Si and ¹³C NMR spectroscopy (Mn=5,642; Mw=13,363).

EXAMPLE 4 Preparation of M^(Vi3) _(0.74)Q_(0.26) resin

30.0 g (207 mmol) of trivinylchlorosilane and 35.2 g (207 mmol) oftetrachlorosilane were mixed into 80 ml of MIBK, and added dropwise intoa solution consisting of 80 ml of a 1M HCl aqueous solution, 120 mltoluene and 160 ml MIBK at room temperature over a 1 h period. Themixture was then refluxed for another 3 hours under constant stirring.The aqueous layer was poured off and the organic layer was washed fourtimes with water until neutral pH. Removal of residual water byanhydrous NaSO₄, and stripping off the solvent led to 17 g of a liquidbeing highly soluble in common organic solvents. The M^(Vi3)_(0.74)Q_(0.26) composition of this resin was determined by ²⁹Si and ¹³CNMR spectroscopy (OH wt%=1.3%; Mn=1,518; Mw=2,170).

EXAMPLE 5 Preparation of M^(Vi3) _(0.56)M^(HMe2) _(0.07)Q_(0.37) resin

16 g of M^(Vi3) _(0.74)Q_(0.26) prepared according to example 4, werere-dissolved into 20 ml of anhydrous toluene and 1.84 g (12.7 mmol) oftrivinylchlorosilane were added. The mixture was stirred at 50° C. for30 min. 0.83 g (6.2 mmol) of 1,1,3,3-tetramethyldisilazane was thenadded in 2 lots at 10 min. interval and the mixture further heated at50° C. for 90 min. After addition of 50ml of distilled water, theorganic layer was collected and washed four times with water untilneutral pH. The mixture was treated by anhydrous NaSO₄ to removeresidual water by further filtration and volatiles were stripped offleading to 5.3 g of a low viscosity yellow liquid (115 mPa.s at 27.4°C.). The M^(Vi3) _(0.56)M^(HMe2) _(0.07)Q_(0.37) composition of thisresin was determined by ²⁹Si and ¹³C NMR spectroscopy (OH wt %<0.6%;Mn=2,295; Mw=10,183).

EXAMPLE 6 Preparation of M^(Vi3) _(0.17)T^(Ph) _(0.30)T^(H)_(0.41)Q_(0.11) resin

75.0 g (355 mmol) of phenyltrichlorosilane, 34.2 g (236 mmol) oftrivinylchlorosilane and 80.1 g (591 mmol) of trichlorosilane were mixedinto 240 ml of MIBK, and added dropwise into a solution consisting of240 ml of a 1M HCl aqueous solution, 360 ml toluene and 480 ml MIBK atroom temperature over a 1h period. The mixture was aged for 3 hours atroom temperature under constant stirring. The aqueous layer was pouredoff and the organic layer was washed four times with water until neutralpH. 300 ml of a 1 M aqueous solution of sodium acetate was added intothe organic layer and the solution was stirred overnight at 400C. Theaqueous phase was poured off and the organic layer was washed four timeswith water until neutral pH. The mixture was treated by anhydrous NaSO₄to remove residual water by further centrifugation. The solvents werestripped off leading to 109 g of a liquid resin (4,000 mPa.s at 27.4°C.). The M^(Vi3) _(0.17)T^(Ph) _(0.30)T^(H) _(0.41)Q_(0.11) compositionof this resin was determined by ²⁹Si and ¹³C NMR spectroscopy (OH wt%=1%; Mn=3,031; Mw=7,652).

EXAMPLE 7 Preparation of M^(Vi3) _(0.16)M^(HMe2) _(0.50)T^(Ph)_(0.29)T^(H) _(0.35)Q_(0.15) resin

69.6 g of M^(Vi3) _(0.17)T^(Ph) _(0.30)T^(H) _(0.41)Q_(0.11) preparedaccording to example 6, were re-dissolved into 600 ml of anhydroustoluene and 1.47 g (15.5 mmol) of dimethylchlorosilane and 2.07 g (15.5mmol) of 1,1,3,3-tetramethyldisilazane were added. The mixture wasstirred at room temperature overnight. The mixture was washed four timeswith water until neutral pH and treated by anhydrous NaSO₄ to removeresidual water by further centrifugation and filtration. The solventswere stripped off leading to a viscous liquid resin (34,000 cP at 26.9°C.). The M^(Vi3) _(0.16)M^(HMe2) _(0.50)T^(Ph) _(0.29)T^(H)_(0.35)Q_(0.15) composition of this resin was determined by ²⁹Si and ¹³CNMR spectroscopy (Mn=2,124; Mw=4,728, OH wt %<0.8%).

EXAMPLE 8 Cure of M^(Vi3) _(0.21)M_(HMe2) _(0.13)T^(Ph) _(0.64) with1,4-bis(dimethylsilyl)benzene

To 5.0 g of M^(Vi3) _(0.21)M_(HMe2) _(0.13)T^(Ph) _(0.64) resin (example2), was added under stirring 2.3 g of 1,4-bis(dimethylsilyl)benzene and0.5 g of a 10 wt % solution of a platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex in toluene(Pt°/SiH=50 ppm). The mixture was sonicated for 20 min. and poured intoa mould for gradual heating up to 200° C. for 3 h. Large pieces ofcrack-free specimen were obtained for analysis by dynamic mechanicalthermal analysis (OMTA) and thermo-mechanical analysis (TMA).

EXAMPLE 9 Cure of M^(Vi3) _(0.21)M_(HMe2) _(0.13)T^(Ph) _(0.64) withM^(H) ₃T^(Ph)

To 4.0 g of M^(Vi3) _(0.21)M_(HMe2) _(0.13)T^(Ph) _(0.64) resin (example2), was added under stirring 2.1 g of M^(H) ₃T^(Ph) and 0.4 g of a 10wt% solution of a platinum (0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex in toluene (Pt°/SiH=50 ppm). The mixture wassonicated for 15 min. and poured into a mould for gradual heating up to200° C. for 3h. The final material was analysed by DMTA and TMA.

EXAMPLE 10 Cure of M^(Vi3) _(0.25)T^(H) _(0.75) with1,4-bis(dimethylsilyl)benzene

To 7.3 g of M^(Vi3) _(0.25)T^(H) _(0.75) resin (example 3), was addedunder stirring for 15 min., 7.8 g of 1,4-bis(dimethylsilyl)benzene and2.7 g of a platinum (0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxanecomplex in toluene (Pt°/SiH=50 ppm). The mixture was sonicated for 15min. and poured into a mould for gradual heating up to 200° C. for 3 h.Large pieces of crack-free specimen were obtained for analysis by DMTAand TMA.

EXAMPLE 11 Cure of M^(Vi3) _(0.56)M^(HMe2) _(0.07)Q_(0.37) with1,4-bis(dimethylsilyl)benzene

To 3.0 g of M^(Vi3) _(0.56)M^(HMe2) _(0.07)Q_(0.37) resin (example 5),was added under string 5.1 g of 1,4-bis(dimethylsilyl)benzene and 1.3 gof a 10 wt% solution of a platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex in toluene(Pt°/SiH=50 ppm). The mixture was sonicated for 5 min. and poured into amould for gradual heating up to 200° C. for 3h. Large pieces ofcrack-free specimen were obtained for analysis by DMTA and TMA.

EXAMPLE 12 Cure of M^(Vi3) _(0.17)T^(Ph) _(0.30)T^(H) _(0.41)Q_(0.11)benzene

To 3.0 g of M^(Vi3) _(0.17)T^(Ph) _(0.30)T^(H) _(0.41)Q_(0.11) resin(example 5) was added under stirring 1.64 g of1,4-bis(dimethylsilyl)benzene and 0.38 ml of a platinum(0)-1,3-divinyl-1,1,3,3-tetramethyl disiloxane complex solution intoluene (Pt°/SiH=50 ppm). The mixture was sonicated and poured into amould for gradual heating up to 200° C. for 3h. The final material wasanalysed by DMTA and TMA.

EXAMPLE 13 Self-addition cure of M^(Vi3) _(0.17)T^(Ph) _(0.30)T^(H)_(0.41)Q_(0.11) resin EXAMPLE 14 Self-addition cure of M^(Vi3)_(0.16)M^(HMe2) _(0.05)T^(Ph) _(0.29)T^(H) _(0.35)Q_(0.15) resin

Self-addition curable silicone resins (examples 6 and 7) were subjectedto addition cure using a platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex in toluene as thecatalyst. The resin was dissolved in anhydrous toluene and then mixedwith a catalytic amount of a platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex in toluene(Pt°/SiH=50 ppm) for 10 minutes to a 75 w t% solution before castinginto a mould. The samples were then heated gradually up to 200° C. for 3h. Large pieces of crack-free specimen were obtained for analysis byDMTA and TMA.

The cured resins produced by each of Examples 1a to 9a were analysed byDMTA and thermo-mechanical analysis TMA and the results are shown inTable 1, in which E′₂₅ is the modulus at 25° C. or Young's modulus andE'_(p) is the plateau modulus (minimum value of Young's modulus overtemperature range −100° C. to +300° C.). The CTE of the cured resins wasmeasured by TMA over various temperature ranges shown in ° C. TABLE 1E′₂₅ E′_(p) CTE Entry (MPa) (MPa) (ppm/K) (temp. range in ° C.) Example8  2,660 590 55 (−80/−40), 55 (−30/0), 81(0/70), 97 (80/140), 134(140/170), 193 (180/200) Example 9  1,870 680 59 (−80/−40), 70 (−30/0),90 (0/100), 184 (120/160) Example 10 2,410 1,090 65 (−80/−40), 73(−30/0), 96 (0/70), 122 (80/140), 152 (140/165) Example 11 2,600 766 62(−80/−40), 75 (−30/0), 87 (0/70), 106 (70/140), 144 (140/170), 127(210/240) Example 12 2,380 960 7 (−80/0), 30 (20/60), 70 (70/100), 154(135/160) Example 13 1,700 1060 71 (−80/−40), 85 (−30/0), 103 (0/70), 96(80/150), 112 (160/190) Example 14 1,920 1,150 73 (−80/−40), 84 (−30/0),107 (0/45), 183 (130/170), 130 (170/195)

EXAMPLES 15 TO 17 Thermal Post-Cure

Free-standing resin films produced according to Examples 12 to 14 weresubjected to an annealing treatment. The addition-cured free-standingresin film was placed into the chamber of a furnace. The chamber waspurged by 3 vacuum/N2 cycles. The samples were then heated under N₂gradually up to 400° C. The N2 inlet was then bubbling into atriethylamine solution and the samples were further heated at 400° C.for 2 hours under a N2/triethylamine vapour atmosphere. Crack-freespecimen were obtained for analysis by DMTA and TMA (Table 2). TABLE 2Cured resin of E′₂₅ E′_(p) CTE (ppm/° K.) Entry Example No. (MPa) (MPa)(temp. range in ° C.) Example 15 12 2900 2190 49 (−80/−40), 65 (−30/0),66 (0/70), 86 (80/140), 93 (140/170) Example 16 13 3470 3380 50(−80/−40), 53 (−30/0), 54 (0/70), 55 (80/140), 65 (140/165) Example 1714 2890 2750 23 (−80/−40), 32 (−30/0), 38 (0/70), 51 (80/140), 60(140/170)

As can be seen from Table 2, the cured resins produced by post-curing at400° C. had very high Young's modulus, very low CTE and showed aparticularly high plateau modulus (negligible decrease in modulus athigher temperature).

1. A curable silicone resin having the empirical formula(R₃SiO_(1/2))_(a)(R₂SiO_(2/2))_(b)(RSiO_(3/2))_(c)(SiO_(4/2))_(d)wherein each r is a hydrocarbon or substituted hydrocarbon group or ahydrogen atom; and a=0.02 to 0.8; b=0 to 0.4; and c+d=0.2 to 0.98, wherea+b+c+d=1.0, and where at least 2 mole % of the siloxane units in theresin are of the formula R′₃SiO_(1/2), RR′2SiO1_(/2 or) R′₂SiO_(2/2),wherein each R′ is an alkenyl group:
 2. A curable silicone resinaccording to claim 1, wherein each R′ is a vinyl group.
 3. A curablesilicone resin according to claim 1, wherein at least 10 mole % of thesiloxane units of the resin are Vi₃SiO_(1/2) groups, where Vi representsvinyl.
 4. A curable silicone resin according to claim 1, wherein atleast 80 mole % of the siloxane units of the resin are selected fromR′₃SiO_(1/2), RSiO3_(/2) and SiO_(4/2) units.
 5. A curable siliconeresin according to claim 1, wherein at least 20 mole % of the siloxaneunits of the resin are ArSiO_(3/2) units where Ar represents an arylgroup.
 6. A self-curable silicone resin according to claim 1, wherein10-50 mole % of the siloxane units of the resin are HSiO_(3/2) units. 7.A self-curable silicone resin according to claim 1, wherein 10-50 mole %of the siloxane units of the resin are HR₂SiO_(1/2), H₂RSiO_(1/2) orHRSiO_(2/2) units.
 8. A curable resin composition comprising (I) acurable silicone resin having the empirical formula(R₃SiO_(1/2))_(a)(R₂SiO_(2/2))_(b)(RSiO_(3/2))_(c)(SiO_(4/2))_(d)wherein each R is a hydrocarbon or substituted hydrocarbon group or ahydrogen atom, and a=0.02 to 0.8; b=0 to 0.4; and c+d=0.2 to 0.98, wherea+b+c+d=1.0, where at least 2 mole % of the siloxane units in the resinare of the formula R′₃SiO_(1/2), RR′2SiO1_(/2 or) R′₂SiO_(2/2), whereineach R′is an alkenyl group and (II) a curing agent having at least onegroup reactive with the alkenyl group R′.
 9. A curable resin compositionaccording to claim 8 wherein the curing agent contains at least one Si—Hgroup and the composition includes a catalyst containing a platinumgroup metal.
 10. A curable resin composition according to claim 9wherein the curing agent is a polysiloxane containing at least two Si—Hgroups or an aryl compound of the formula HX₂Si—Ar—SiX₂H, in which Ar isa substantially nonflexible linkage including at least one para-arylenemoiety and each X is a hydrocarbon or substituted hydrocarbon group or ahydrogen atom.
 11. A curable resin composition comprising a self-curableresin according to claim 6 and a catalyst containing a platinum groupmetal.
 12. A process for the preparation of a cured heat resistantsilicone resin having a low coefficient of thermal expansion, comprisingthe step of reacting a curable silicone resin having the empiricalformula(R₃SiO_(1/2))_(a)(R₂SiO_(2/2))_(b)(RSiO_(3/2))_(c)(SiO_(4/2))_(d)wherein each R is a hydrocarbon or substituted hydrocarbon group or ahydrogen atom; and a=0.02 to 0.8; b=0 to 0.4; and c+d=0.2 to 0.98, wherea+b+c+d=1.0, where at least 2 mole % of the siloxane units in the resinare of the formula R′₃SiO_(1/2), RR′2SiO1_(/2 or) R′₂SiO_(2/2), whereineach R′ is an alkenyl group, with a curing agent having at least onefunctional group reactive with the alkenyl group R′.
 13. A processaccording to claim 12, wherein the curing agent contains at least oneSi—H group and the curing process is carried out in the presence of acatalyst containing a platinum group metal.
 14. A process for thepreparation of a cured heat resistant silicone resin having a lowcoefficient of thermal expansion, comprising the step of heating aself-curable silicone resin according to claim 6 in the presence of acatalyst containing a platinum group metal.
 15. A process forencapsulating a substrate comprising the steps of coating the substratewith a curable silicone resin composition comprising (I) a curablesilicone resin having the empirical formula(R₃SiO_(1/2))_(a)(R₂SiO_(2/2))_(b)(RSiO_(3/2))_(c)(SiO_(4/2))_(d)wherein each R is a hydrocarbon or substituted hydrocarbon group or ahydrogen atom and a=0.02 to 0.8; b=0 to 0.4; and c+d=0.2 to 0.98, wherea+b+c+d=1.0, where at least 2 mole % of the siloxane units in the resinare ofthe formula R′₃SiO_(1/2), RR′2SiO1_(/2 or) R′₂SiO_(2/2), whereineach R′ is an alkenyl group, and (II) a curing agent having at least onegroup reactive with the alkenyl group R′, and then reacting (I) and(II).
 16. A process for coating a substrate comprising the steps ofapplying a curable silicone composition comprising (I) a curablesilicone resin having the empirical formula(R₃SiO_(1/2))_(a)(R₂SiO_(2/2))_(b)(RSiO_(3/2))_(c)(SiO_(4/2))_(d)wherein each R is a hydrocarbon or substituted hydrocarbon group or ahydrogen atom; and a=0.02 to 0.8; b=0 to 0.4: and c+d=0.2 to 0.98, wherea+b+c+d=1.0, where at least 2 mole % of the siloxane units in the resinare of the formula R′₃SiO_(1/2), RR′2SiO1_(/2 or) R′₂SiO_(2/2), whereineach R′ is an alkenyl group and (II) a curing agent having at least onegroup reactive with the alkenyl group R′ as a thin film to a substrateand then reacting (I) and (II).
 17. A process for making a compositematerial, comprising the step of impregnating at least one layer offibrous material with a curable silicone composition comprising (T) acurable silicone resin having the empirical formula(R₃SiO_(1/2))_(a)(R₂SiO_(2/2))_(b)(RSiO_(3/2))_(c)(SiO_(4/2))_(d)wherein each R is a hydrocarbon or substituted hydrocarbon group or ahydrogen atom; and a=0.02 to 0.8; b=0 to 0.4; and c+d=0.2 to 0.98, wherea+b+c+d=1.0, least 2 mole % of the siloxane units in the resin are ofthe formula R′₃SiO_(1/2), RR′2SiO1_(/2 or) R′₂SiO_(2/2), wherein each R′is an alkenyl group, and (II) a curing agent having at least one groupreactive with the alkenyl group R′ and then reacting (I) and (II).
 18. Aprocess according to claim 12, wherein the first step is at atemperature in the range 50 to 300° C. and further comprising asubsequent heat cure step at a temperature in the range 300 to 500° C.19. A process according to claim 18, wherein the further heating step at300-500° C. is carried out in the presence of an amine which is in thevapour state at the temperature of the further heating step.
 20. Aprocess according to claim 19, wherein the amine is a tertiary amine ofthe formula NZ₃, where each Z represents an alkyl group having 1 to 4carbon atoms.
 21. A cured heat resistant silicone resin compositionprepared by the process of claim
 12. 22. A cured heat resistant siliconeresin composition prepared by the process of claim 18.